1. Field of the Invention
An operator-manipulable pointing device, e.g., a digitizer pen, which includes a digitizer stylus for touching a sensing surface, has a built-in pressure sensor that is automatically activated when the tip of the stylus applies a force on the sensing surface.
2. Description of the Related Art
Digitizer pens that include a stylus, which develop a signal output responsive to a longitudinal force on a writing tip of the stylus and which have a conductive member adjacent to a resistive member within the stylus, are well known. Such digitizer pens have a pressure sensor at an end of the stylus opposite the writing tip. Such pressure sensor includes a compression means for forcing the resistive member and the conductive member together in electrical contact over a contacting area that is directly proportional to an amount of longitudinal pressure of the writing tip. Such pressure sensor has two terminals, and the electrical conductance or electrical resistance between the two terminals constitutes an output of the pressure sensor.
For example, U.S. Pat. No. 5,357,062 entitled PRESSURE SENSING APPARATUS FOR DIGITIZER PEN TIP issued Oct. 18, 1994 to Rockwell et al., discloses a resistive member made from a carbon-impregnated elastomeric material, and a conductive member that includes a pair of rectangular-shaped, conductive silver ink traces on an insulating substrate. The aforementioned patent also discloses a conductive member comprising a piece of flexible rubber having a conductive coating, and a resistive member that includes a pair of triangular-shaped, carbon/silver ink traces on an insulating substrate. The triangular-shaped carbon/silver ink traces have their triangular apexes in adjacent proximity. The end of the stylus opposite the writing tip must be centered between the apexes of the triangular-shaped ink traces because when the end is off-centered, the output of the prior art pressure sensor is, disadvantageously, substantially different from the output when the end is centered.
Prior art pens have a known problem with the centering of the conductive ink traces or the resistive ink traces, or both, on their respective substrates during the printing process. Typical printing tolerance is ±5 mils. Prior art pens also have a known problem with the centering (with respect to the axis of the stylus) of the conductive ink traces or the resistive ink traces, or both, during assembly of the pen. Looking at the latter problem another way, prior art pens have a known problem with the centering of the stylus between the apexes of the two triangular-shaped conductive ink traces or the two triangular-shaped resistive ink traces, or both, during assembly of the pen. Prior art pens have another known problem of a properly assembled stylus becoming off-centered with respect to the apexes of the triangular-shaped ink traces, as a result of wear or misuse. Prior art pens also have the problem of a properly assembled stylus becoming off-centered with respect to the apexes of the triangular-shaped ink traces because of an accumulation of manufacturing tolerances. As a result, with standard manufacturing tolerances, some prior art digitizer pens would not function after having been seemingly properly assembled. In any case, prior art pens have a problem of the end of the stylus opposite the writing tip becoming off-centered with respect to ink traces on the pressure sensor.
With known digitizer pens, if the pressure to the pressure sensor is applied off-centered for one or more of the aforesaid reasons, a start-up characteristic occurs that is disadvantageously different from the start-up characteristic that occurs when the pressure is applied to the center of the pressure sensor.
Known pressure sensors for digitizer pens that use a force-sensitive, resistive ink, use a carbon-type ink. Disadvantageously, carbon-type inks tend to flake off a substrate. Carbon-type inks are formulated with other inks to produce a desired resistivity, and control of the formulation is very difficult.